It has been a longstanding problem in the design and construction of helicopters, that the rotor drive train and auxiliary devices as well as the main rotor and the tail rotor generate a substantial noise load in the interior of the cabin of the helicopter. In order to reduce this noise loading, it has become known to cover or panel the interior walls of the helicopter cabin with noise damping panels or liners. However, the use of such noise damping panels or the like entails a very substantial effort and expense in terms of the installation and construction thereof, and also causes a substantial weight penalty in the helicopter. This is especially true if the noise damping elements are to be effective over the various noise frequency ranges of noise generated by the various above mentioned helicopter noise sources.
Another approach to noise attenuation is known from the publication "AGARD CONFERENCE PROCEEDINGS 549" "Impact of Acoustic Loads on Aircraft Structures" (September 1994) in an article by G. Niesl et al., entitled "Helicopter Internal noise ", which describes a helicopter wall structure having a sandwich construction type with an integrated noise insulation. In the known sandwich structure, the fiber composite cover layer that is arranged facing toward the source of noise is provided with a plurality of through-going holes that lead to the individual honeycomb cells of the sandwich core. By providing such holes through the noise-loaded cover layer, the honeycomb cells are thereby embodied to act as noise absorbing cavities in a discrete frequency range in the manner of a Helmholtz-resonator.
However, such a known construction suffers disadvantages, for example each individual through-going hole causes fiber breaks or interruptions in the fiber composite material of the cover skin. As a result, the specific strength of the sandwich structure relative to the surface or area weight thereof, becomes significantly reduced compared to a comparable sandwich structure without such through-going holes, especially if the density of holes is relatively large. Moreover, boring the individual holes through the cover layer requires an increased effort and expense in fabricating the composite structural panel. Furthermore, the open holes provide an undesirable access path for various environmental influences, such as moisture and dust deposits and the like, to penetrate into the interior of the sandwich structure, which increases the weight of the structure, reduces the noise absorbing performance over time, and leads to the accelerated degradation of the structure.